Mobile elevating work platforms generally have a cage for an operator that is mounted on the end of a hydraulically-operated boom. The boom is mounted on a base unit and can be moved up and down or from side-to-side under the control of an operator in the cage.
The base unit is provided with drive wheels and is fully mobile. It is stabilised by the weight of the base unit and a counter-weight attached to the boom.
In one known type of MEWP the chassis of the base unit is entirely rigid: the wheels are mounted on rigid axles without any form of suspension and have very hard tyres. This provides the stability to ensure that the base unit does not tilt when the boom is extended to one side of the base unit. The base unit is very heavy, and the wheels and tyres provide a significant part of the weight of the base unit.
One disadvantage of using a rigid chassis is that when the MEWP is standing on an uneven surface one of the wheels may be raised off the ground. This does not seriously affect the stability of the platform as the weight of the lifted wheel still contributes to the overall weight of the base unit. However, having one wheel out of contact with the ground can be disconcerting for the operator. Also, if the lifted wheel is a drive wheel, drive from that wheel will be lost.
GB2334015A describes a MEWP in which the base unit includes a suspension mechanism that includes, for each drive wheel, a swing arm on which the wheel is mounted. The swing arm is arranged to pivot about a substantially horizontal pivot axis and a stop member is provided on the chassis that engages the swing arm to limit upwards movement. The swing arm engages the stop members when the mobile access platform is standing on flat, level ground and the platform cannot therefore tilt significantly, ensuring stability.
A biasing spring is connected to each swing arm, which biases the swing arm upwards towards the stop member. The biasing force is however slightly less than the force needed to maintain the swing arm in engagement with the stop member when the wheel is unsupported. Therefore, when the MEWP is standing on uneven ground, one or other of the swing arms can pivot downwards from its normal position in engagement with the stop member, allowing the unsupported wheel to drop into contact with the ground. All four wheels can therefore maintain contact with the ground even when the platform is standing on or travelling over uneven ground. This improves the confidence of the operator and maintains drive from both of the drive wheels. Also, as the swing arm is biased upwards towards the stop member, the weight of that arm and the associated wheel contribute to the overall weight of the base unit and to the stability of the platform.
The force that the dropped wheel exerts on the ground can be controlled by adjusting the biasing force applied by the spring. Normally, the spring is adjusted so that most of the weight of the dropped wheel and the swing arm is carried by the spring, so that this weight contributes to the stability of the MEWP. This means however that the wheel exerts only a small force on the ground, which can cause it to slip when drive is applied. Reducing the force applied by the spring will increase the force exerted by the wheel on the ground and reduce the risk of slip, but will also reduce the stability of the MEWP. It may therefore be difficult to achieve an ideal compromise between these two conflicting requirements. Furthermore, the fact that the swing arms can pivot downwards from their normal positions in engagement with the stop members may lead to a perceived, but incorrect, impression that the platform has become unstable.
It is an object of the present invention to provide a base unit for a vehicle that mitigates at least one of the afore-mentioned disadvantages, or that provides one or more advantages over the previous arrangement.